1. Field of the Invention
The invention pertains to the art of oil return systems for centrifugal refrigerant gas compressors.
2. Description of the Prior Art and Additional Background
U.S. Pat. Nos. 3,927,889 and 3,927,890 disclose improved seal arrangements for inhibiting the leakage of refrigerant gas from the compressor discharge space to the compartment containing the gears and bearings for driving the compressor impeller located in the compressed gas collecting scroll.
Centrifugal refrigerant compressors of the type disclosed in those patents, and typically used in liquid chiller packages for example, are arranged in an overall hermetic system in which the gear and bearing compartment or housing is vented to the suction space upstream of the impeller. Those patents should be referred to for somewhat detailed reasons for attempting to provide a highly effective seal between the high pressure discharge space and gear and bearing housing. However, for purposes of this patent application it may be stated that centrifugal compressors of this type should provide venting to the suction side of the impeller or else the pressure in the gear and bearing housing will build to a level that oil may be driven or blown into the motor box containing the electric motor which drives the shafts and gears.
As pointed out in the patents, the centrifugal compressor assemblies of the type of concern in this patent application is of a design in which compactness is considered an important feature. This is achieved in part by the use of a relatively small, high speed compressor arrangement in which the compressor impeller is driven at nominal speeds of 34,000 RPM. In obtaining overall compactness of the compressor assembly, the gear and bearing housing is also relatively compact so that the oil capacity contained in the housing is limited. For example with a centrifugal compressor of the compact size having approximately 100 tons refrigeration producing capacity, the oil capacity is approximately 2.8 gallons initially residing in the sump of the housing. The small housing also means the internal volume and internal surface area in the housing are relatively limited. Thus the total amount of oil mist which circulates in the housing and then drops back into the sump is somewhat limited. That oil which is not on a surface or in the sump is subject to being entrained by the refrigerant gas being vented back to the refrigerant system at a rate of carryout generally proportional to the refrigerant venting rate. The oil carried out is separated from the gas and stored in an accumulator. In time the oil in the accumulator reaches a volume that a shutdown of the system is required to permit drainage of the oil from the accumulator back to the gear and bearing housing. In the noted patents it is pointed out that the seal arrangements of those inventions are intended to substantially reduce leakage to minimize the oil carryout to lengthen the time between required shutdowns.
The approach of my invention is based upon the concept that while the centrifugal refrigeration compressor and system are intended to be designed so that oil does not circulate with the refrigerant, oil in any event will be lost to the refrigerant through leaking O-ring seals, gaskets, refrigerant cooled oil coolers, casting porosity, and so on, and that as a practical matter it is very difficult to have a perfectly leak-free unit without undue cost. Hence, recognizing this to be the case, my invention contemplates a system in which provision is made throughout the system for accomplishing the continuous return of the oil to the gear and bearing housing, with a key part of the system being a relatively small, highly efficient oil and gas separator or filter of a type not heretofore used in this art and incorporated in a system to provide continuous return of oil